HTML不常用元素：optgroup-小小僵尸

 1 <select>
 2     <optgroup label="Group 1">
 3         <option>Option 1.1option>
 4     optgroup>
 5     <optgroup label="Group 2">
 6         <option>Option 2.1option>
 7         <option>Option 2.2option>
 8     optgroup>
 9     <optgroup label="Group 3" disabled>
10         <option>Option 3.1option>
11         <option>Option 3.2option>
12         <option>Option 3.3option>
13     optgroup>
14 select>